Hybrid vehicle fuel systems may include a sealed fuel tank configured to withstand high fuel tank pressure and vacuum levels. Fuel tank pressure or vacuum levels may build up due to engine operating conditions as well as the generation of diurnal vapors over vehicle drive cycles. During refueling of a fuel tank, a fuel door may be maintained locked until sufficient depressurization has occurred to disable refueling of a pressurized fuel tank. A fuel tank pressure sensor may be used to determine if there is excess pressure or vacuum build-up in the fuel tank, and if depressurization is required. During the depressurization, fuel tank vapors may be released into and stored in a fuel vapor canister packed with an adsorbent by opening a valve coupled between the fuel tank and the canister. At a later time, when the engine is in operation, the stored vapors can be purged into the engine intake manifold for use as fuel.
One example approach for verifying fuel tank depressurization is shown by Wolber et al. in US 2007/0101973. Therein, a fuel system pressure is inferred based on engine conditions when an engine is turned off relative to engine conditions when the engine is started during a subsequent driving cycle. For example, based on a difference between a shut-down fuel system temperature and a starting fuel system temperature, a depressurized fuel system may be inferred. In addition, the fuel tank pressure sensor may be diagnosed based on the fuel system temperature change.
However, the inventors herein have recognized potential issues with such systems. As one example, in hybrid vehicles have drastically reduced engine running times (such as PHEVs), fuel system temperatures may not vary enough to correctly infer fuel tank depressurization. If the fuel system pressure sensor is degraded, the fuel tank temperature may not be reliably used to determine if sufficient depressurization has occurred. If depressurization is not correctly determined, the fuel door may remain locked and the operator may not be able to refill the fuel tank. Alternatively, the operator may get showered with fuel mist if the fuel door is opened prematurely.
In one example, some of the above issues may be addressed by a method for a fuel system coupled in a hybrid vehicle. The method may comprise, during refueling conditions, directing fuel tank vapors to an engine intake manifold, and indicating depressurization of a fuel tank based on an output of a mass air flow sensor coupled to the intake manifold. In this way, depressurization may be reliably determined even if a fuel tank pressure sensor goes bad.
For example, an operator may indicate a refueling request by actuating a refueling button on a dashboard of a hybrid vehicle. In response to the refueling request, a fuel tank pressure may be estimated by a fuel tank pressure sensor. If the estimated fuel tank pressure (or vacuum) is above a threshold, depressurization may be required prior to unlocking a fuel door and enabling refueling. As a backup method, to compensate for any malfunction of the fuel tank pressure sensor (e.g., the sensor being degraded or stuck in range), fuel tank depressurization may be inferred from manifold flow. Specifically, in response to the refueling request, the fuel system may be sealed by closing a canister vent valve (CVV) coupling a fuel system canister to the atmosphere) while a fuel tank isolation valve (FTIV) coupling the fuel tank to the fuel system canister, and a canister purge valve (CPV) coupling the canister to an engine intake manifold are opened to relieve the elevated fuel tank pressure. An intake throttle valve may also be commanded open. Due to the specific valve adjustments, the only path for tank pressure (or vacuum) relief is through the CPV into the engine intake. If there is any tank pressure or vacuum to relieve, an air or vapor flow will occur through the engine intake which may be detected by an intake manifold mass air flow (MAF) sensor. Therefore, in response to an output from the MAF sensor (e.g, an output that is higher than a threshold, or any output), it may be inferred that fuel tank depressurization is ongoing and a fuel door may be maintained locked. As such, fuel tank vapors may continue to be diverted to the engine intake until the fuel tank has completely depressurized to ambient pressure conditions (e.g., to barometric pressure conditions). In response to the output from the MAF sensor being lower than the threshold, or when the MAF sensor stops responding, it may be inferred that fuel tank depressurization is complete and that it is safe to refuel. At this time, the fuel door may be unlocked and the vehicle operator may be able to refill the tank from an external fuel source.
In some embodiments, degradation of the fuel tank pressure sensor may also be determined based on discrepancies between the output of the fuel tank pressure sensor and the MAF sensor. For example, during refueling conditions when an output of the fuel tank pressure sensor is lower than a threshold, indicating absence of excess fuel tank pressure, but the output of the MAF sensor is higher than a threshold, indicating depressurization flow of fuel tank vapors to the engine intake, it may be determined that the fuel tank pressure sensor is degraded and a diagnostic code may be set.
In this way, an auxiliary method is provided for verifying fuel tank depressurization during refueling. By diverting excess fuel tank pressure or vacuum to an engine intake, tank depressurization may be inferred based on flow of air or vapors from the sealed fuel tank to the engine intake. By using an existing MAF sensor to detect the flow, the need for additional hardware for detecting depressurization is reduced. By relying on the output of a fuel tank pressure sensor and a MAF sensor to detect depressurization, the reliability of the results is improved. In addition, depressurization can be accurately identified using the MAF sensor even when the fuel tank pressure sensor is degraded. By holding the fuel door locked until fuel tank depressurization is confirmed, the vehicle operator may be protected from fuel spray. In addition, evaporative hydrocarbon emissions (or leakage) is reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.